1. Field of the Invention
The invention is concerned with non-metallic fiber reinforcement of formable matrices used as structural material, such as concrete, plaster, and sand-lime mixtures. It is particularly concerned with controlling the deterioration of such structural materials by alternate freezing and thawing, as well as improvement in flexural strength.
2. Description of the Prior Art
The relative weakness of certain hydraulic setting matrices used as structural materials is well known. For example, concrete is strong in compression but relatively weak in tension and therefore in flexure. It is common for this reason to use steel reinforcement in concrete, and prestressing or post-stressing of the steel places the concrete in compression to counteract flexural or tension loading.
Freeze-thaw cycles cause disruption of concrete for the same reason. Freezing of minute particles of ice inside the concrete cause tensile forces which are in excess of its ability to withstand, causing portions (near the surface particularly) to spall.
It is known to admix fibers with the matrix to counteract these drawbacks. Among disclosures of steel or wire admixed fibers are the following:
Romualdi-- U.S. Pat. No. 3,429,094
Ball et al-- U.S. Pat. No. 3,650,785
National Standard Co., Technical Data Bulletin No. SWT-114, September, 1970.
The use of wire or steel fibers is unsatisfactory for several reasons. Steel has low elongation even at high tensile stresses and the fibers are likely to slip within the matrix when the structural member is subjected to loads. In addition to the relative stiffness and lack of workability of steel, it is relatively heavy and therefore costly to achieve a given proportionate volume within the matrix.
The use of non-metallic fibers admixed in matrices for strengthening purposes is equally well known. Examples of patents disclosing such compositions are the following:
Jarboe-- U.S. Pat. No. 3,044,547
Shannon-- U.S. Pat. No. 3,147,127
Fischer et al-- U.S. Pat. No. 3,533,203
Zonsveld-- U.S. Pat. No. 3,591,395
Goldfein-- U.S. Pat. No. 3,645,961
Howe-- U.S. Pat. No. 3,679,445
Kempster-- U.S. Pat. No. 3,716,386
Kurtz-- U.S. Pat. No. 3,769,052
A characteristic of these fibers which creates a drawback is that under stress they begin to stretch, and in stretching their diameter shrinks. By shrinking, the fibers may lose their bond with respect to the matrix. This sequence causes rapid progressive failure when the yield strength of the composite fiber-matrix mixture is reached.
The same would be true of conventional reinforcing rod, except that it is preformed before use to provide mechanical connections with the matrix that persist regardless of shrinkage in cross section. This is not possible with previous extruded fibers which are merely cut off clean. A deformed fiber, such as some of the available metal ones or others (see National Standard Co. Specification No. SWT-114 and Howe U.S. Pat. No. 3,679,445 ) which have intermittent flattened or twisted sections, or crimped, hammered or tapered profiles, does not counteract this drawback. Other patents in this same general area discovered while conducting searches on the subject matter of this invention are the following:
Fischer et al-- U.S. Pat. No. 3,208,838
Kjell-Berger-- U.S. Pat. No. 3,217,075
Hull et al-- U.S. Pat. No. 3,466,822
Gothard et al-- U.S. Pat. No. 3,637,457
Campbell-- U.S. Pat. No. 3,676,968
The co-pending application of Terence M. Shaw, Ser. No. 554,162, filed Feb. 28, 1975 and assigned to the assignee of the present application now U.S. Pat. No. 3,980,484, shows a fiber reinforced structural material in which the fibers have enlarged knob-like ends serving as anchors to enhance the bonding action between the fibers and the matrix. Karl V. Hauser's co-pending application Ser. No. 513,321, filed Oct. 9, 1974, now abandoned, shows another fiber reinforced structural material in which each fiber is so curved that the matrix, under stress, will exert forces in different directions along its length. Fibers with the cross-sectional shape shown in this application have been used for paintbrush bristles for the purpose of obtaining more paint-carrying capacity. The present invention is believed patentable over these prior disclosures, as well as over the references cited in the co-pending Hauser application.